It is known to provide in such a central office a processor which, besides facilitating the establishment of a connection between subscribers in response to selection signals such as trains of dial pulses, monitors the evolution of that connection and registers certain information pertaining thereto, especially the duration and/or the charge therefor in the case of a toll call. That information, however, is generally fed into a centralized computer and is not readily available to a calling (or called) subscriber, except by means of complicated and therefore expensive data-transmission equipment not invariably included in such systems.